President Bush issued a formal national security directive yesterday ordering agencies to prepare contingency plans for a surprise, "decapitating" attack on the federal government, and assigned responsibility for coordinating such plans to the White House.

The prospect of a nuclear bomb being detonated in Washington without warning, whether smuggled in by terrorists or a foreign government, has been cited by many security analysts as a rising concern since the Sept. 11, 2001, attacks.

The order makes explicit that the focus of federal worst-case planning involves a covert nuclear attack against the nation's capital, in contrast with Cold War assumptions that a long-range strike would be preceded by a notice of minutes or hours as missiles were fueled and launched.

"As a result of the asymmetric threat environment, adequate warning of potential emergencies that could pose a significant risk to the homeland might not be available, and therefore all continuity planning shall be based on the assumption that no such warning will be received," states the 72-paragraph order. It is designated National Security Presidential Directive 51 and Homeland Security Presidential Directive 20.

The statement added, "Emphasis will be placed upon geographic dispersion of leadership, staff, and infrastructure in order to increase survivability and maintain uninterrupted Government Functions."

After the 2001 attacks, Bush assigned about 100 senior civilian managers to rotate secretly to locations outside of Washington for weeks or months at a time to ensure the nation's survival, a shadow government that evolved based on long-standing "continuity of operations plans."

Since then, other agencies including the Pentagon, the Office of the Director of National Intelligence and the CIA have taken steps to relocate facilities or key functions outside of Washington for their own reasons, citing factors such as economics or the importance of avoiding Beltway "group-think."

Norman J. Ornstein, a scholar at the American Enterprise Institute and an adviser to an independent Continuity of Government Commission, said the order "is a more explicit embrace of what has been since 9/11 an implicit but fairly clear set of assumptions."

He added, "My frustration is that those assumptions have not gripped the Congress in the same way."

Other former Bush administration officials said the directive formalizes a shift of authority away from the Department of Homeland Security to the White House.

Under an executive order dating to the Reagan administration, responsibility for coordinating, implementing and exercising such plans was originally charged to the Federal Emergency Management Agency and later DHS, the Congressional Research Service noted in a 2005 report on a pending DHS reorganization.

The new directive gives the job of coordinating policy to the president's assistant for homeland security and counterterrorism -- Frances Fragos Townsend, who will assume the title of national continuity coordinator -- in consultation with Bush's national security adviser, Stephen J. Hadley, with the support of the White House's Homeland Security Council staff. Townsend is to produce an implementation plan within 90 days. Homeland Security Secretary Michael Chertoff will continue to coordinate operations and activities, the directive said.